1. Field of the Invention
The invention relates to a method of forming micro solder balls for bonding integrated circuits to a module substrate or to a circuit board, using a C4 bonding process. In particular, there is described a method for forming micro solder balls by a tape liftoff process which allows for an increased number and density of input/ouput connections to be fabricated on each integrated circuit chip, module substrate or circuit board.
2. Description of the Related Art
The central processing units (CPUs) of most modern day computers are typically provided on large circuit boards (mother boards) populated with various integrated circuit (IC) chips, such as microprocessor chips and memory chips. These IC chips work in conjunction with one another to perform the functions of the computer. Contacts on the mother board are connected to contacts on the IC chips by the use of multi-chip modules or directly by conventional means, such as solder. The chips are connected to one another by metal patterns formed on the surface of the module or the substrate mother board. These metal patterns provide a conduit for data exchange between the IC chips.
There is a constant need for computers which operate at faster rates. In order to accommodate this need, various techniques have developed to increase the rate (bandwidth) at which data can be processed and transmitted. One of these involves increasing the circuit complexity of IC circuits which also often results in a larger package for the IC chip and an increase in the number of input/output (I/O) terminals for chip. Since the amount of data that can be accessed from or transferred to an IC chip is directly proportional to the number of I/O lines the chip contains, increasing the number of I/O terminals directly increases data transfer and processing speed.
IC chips have traditionally been packaged in modules before they are bonded to a mother board or other circuit board. The module consists of one or more IC chips bonded to a module substrate. The composite IC chip-module substrate is then bonded to a computer mother board or other circuit board. Although this is the traditional method of attachment, recently IC chips have also been directly bonded to mother boards.
Traditionally, IC chips were connected to the first level metal pattern on a module substrate with fine wires (wire bonding). This method of connection was limited by the number of pads which could be placed on the periphery of the IC chip. Since then, considerable progress has been made in reducing the IC chip pad size, thereby increasing the number of pads. However, this technology is still limited by the number of pads which can be formed on the chip periphery, and therefore the number of I/Os on a chip is likewise limited. Therefore, other techniques have been developed over the past 30 years to increase the number of available I/O terminals and eliminate alignment problems.
One of these techniques, known as Controlled Collapse Chip Connection (C4), was developed in the 1960s to deal with the problems associated with alignment of integrated circuit chips on a substrate of a module. This process also sought to increase the number of available I/O terminals which could be available for each IC chip. The C4 process uses solder bumps deposited on flat contacts on the IC chips to form the bond between the IC chip and the module substrate. The contacts and solder balls on the IC chips are matched with similar flat contacts on the module substrate to form the connection. Once the chip is placed on top of the contacts of the module substrate, the entire device is heated to a temperature which melts the solder. Then, the solder is allowed to set, and a reliable bond is formed between the chip and the module substrate. Although the C4 bonding process is usually employed to bond an IC chip to a module substrate, it can also be used to bond an IC chip directly to a mother board or other circuit board.
One of the main advantages of this process is that the IC chip self-aligns itself on the module substrate based on the high surface tension of the solder. In other words, the chip need not be perfectly aligned over the contacts of the substrate, as long as it is in close proximity the melting of the solder will align the chip with the substrate contacts. The other advantage of this process is that an increased number of I/O terminals can be fabricated for each IC chip. This type of bonding process is also often referred to as "flip-chip", or "micro-bump" bonding. The process can be briefly explained with reference to FIGS. 1 and 2.
FIG. 1 shows a side view of a IC chip 10 and a module substrate 20. The IC chip 10 is fabricated with various metal pattern lines and contacts 50 imprinted on its last metal level, as shown in FIG. 2. Formed beneath the IC chip 10 is an array of solder balls 30. The module substrate 20 includes metallized paths 60 for carrying signals from the IC chip 10 to other elements mounted on the substrate. These paths have contacts which match the contacts located on the underside of the IC chip 10. When the IC chip 10 is ready to be mounted, it is placed on top of the module substrate 20 above the substrate contacts with the solder balls 30 attached to the contacts of the IC chip resting on the contacts of the module substrate, as shown in FIG. 1. When the device is heated, the solder melts and the chip 10 self-aligns with the module substrate contacts. The solder later hardens to form a reliable bond between the two sets of contacts. FIG. 3 shows the device after the solder has been heated and set. The completed module is then bonded to a mother board (not shown) through wire bonds or additional C4 connections at the module terminals.
Traditionally, the contacts and solder balls have been formed on the IC chip using metal mask technology. In this process a metal mask (essentially a metal plate with a pattern of holes therein) is placed over an IC wafer containing many IC chips 20 for forming the contacts and solder balls. Then, contact material and solder are evaporated through the holes onto the wafer. The holes in the metal masks must be of sufficient size to prevent warpage and damage of the mask during use. Hence, the number of contacts that can be fabricated through use of a metal mask is limited because the holes in the mask must remain above a minimum size to prevent these problems. Consequently, the size of the solder balls that can be created is similarly limited.
The minimum diameter of a C4 solder ball that can be achieved using current techniques, such as metal mask, is approximately 100 microns. Since the size of the solder balls is directly related to the number and density of I/O terminals that can be fabricated on a given IC chip, a decrease in solder ball size would provide for an increase in the number and density of the I/O terminals. This would, in turn, allow for a significant increase in data transmission rates because of the increased number of I/O ports for the packaged IC circuit.
Hence, there is currently a need for a process for forming C4 solder balls which are less then 100 microns in diameter.